1. Field of the Invention
The present invention relates to a system for providing exhaust gas recirculation (EGR) for a compression-ignition internal combustion engine which reduces or controls formation of EGR condensate.
2. Background Art
A number of strategies have been developed for alternative charge air handling and turbocharging to drive and control exhaust gas recirculation (EGR) to reduce emissions for truck, automotive, and stationary engines used in power plants. One approach uses a single state variable geometry turbocharger (VGT), in combination with an EGR circuit to achieve the desired ratio of EGR rate and air/fuel ratio under transient and steady-state operation. In this arrangement, the EGR circuit generally includes a modulating (proportional) or on/off EGR valve, an EGR cooler, and an EGR rate measuring device with appropriate tubing or integral passages to direct exhaust gas to the engine intake under appropriate operating conditions. The management of EGR flow is performed by an electronic control unit (ECU). The ECU may use closed loop control of the EGR flow which is dependent on EGR rate measurement. The ECU may also control the VGT and/or EGR valve based on input from the rate measurement device to regulate EGR flow.
The EGR cooler plays an important role in overall emissions control. Recirculated exhaust gas acts as a dilutant to the charge air which also lowers the volumetric efficiency of the engine. This leads to a lower (richer) air/fuel ratio in comparison to a non-EGR engine because the recirculated exhaust gas has less oxygen content than the charge air due to the oxygen being consumed during the previous combustion process. For an EGR engine to maintain the same air/fuel ratio as a non-EGR engine under the same operating conditions generally requires an increased turbo boost which may in turn require an increase in back pressure to drive the recirculated exhaust gas.
The EGR cooler provides a restriction in the EGR circuit which creates a pressure drop that the turbocharger must overcome by generating more boost pressure to create back pressure to drive the EGR flow. Generating this additional boost compared to a non-EGR engine under the same operating conditions puts added demands on the turbocharger. For example, the turbocharger must withstand higher pressure ratios, higher rotational velocities, higher temperatures, and may experience an increased probability of high cycle fatigue. The EGR cooler can also lower the recirculated exhaust gas to such a temperature that results in condensation which is acidic in nature and may lead to premature degradation of various components including the intake manifold and cylinder liner and kits. Fouling or soot accumulation in the EGR cooler can lead to a progressive performance degradation of the cooler by increasing the pressure drop and resulting in a higher air side outlet temperature which may affect engine performance and fuel economy.
An object of the present invention is to provide a system for utilizing EGR in a multi-cylinder compression ignition internal combustion engine.
Another object of the present invention is to provide an EGR system with selective EGR cooler bypass to reduce or eliminate condensation.
A further object of the present invention is to provide an EGR system with a condensation trap to reduce or eliminate component wear due to EGR condensate.
Yet another object of the present invention is to provide an EGR system having a full flow, two-pass EGR cooler.
Another object of the present invention is to reduce EGR component fouling by maintaining a high EGR mass flow velocity.
A further object of the present invention is to avoid localized boiling within the EGR cooler under conditions providing low coolant flow and high EGR flow.
In carrying out the above objects and other objects, features, and advantages of the present invention, a system for providing exhaust gas recirculation in a multi-cylinder compression ignition internal combustion engine includes an EGR valve in communication with an exhaust side of the engine to selectively divert a portion of the exhaust through an EGR circuit to an intake side of the engine and a two-pass, full flow EGR cooler disposed within the EGR circuit having a cross-sectional area sized to increase EGR flow rates and reduce fouling. In one embodiment, a bypass valve is positioned downstream of the EGR valve and upstream of the EGR cooler to selectively divert at least a portion of recirculated exhaust gas around the EGR cooler based on engine operating conditions to reduce or eliminate condensation of the recirculated exhaust gas. A condensation trap may be positioned downstream of the EGR cooler to collect any EGR condensate which is subsequently vaporized using an associated electric heater having appropriate piping to bypass the turbocharger and deliver the gaseous mixture to the tailpipe. The EGR cooler bypass may be used alone or in combination with the condensation trap depending upon the particular application. A charge air cooler bypass may also be provided for selectively bypassing the charge air cooler for a portion or all of the charge air from the turbocharger before being mixed with the EGR flow to reduce or eliminate condensation in the intake manifold. The charge air cooler bypass may be used alone or in combination with the EGR cooler bypass and/or one or more condensation traps and associated heater.
The present invention provides a number of advantages relative to the prior art. For example, the present invention provides an EGR strategy which utilizes increased EGR mass flow to reduce fouling of EGR components. The use of a full flow EGR cooler which receives full coolant flow from the engine water pump increases the cooling capacity and reduces the potential for localized boiling. An EGR cooler bypass used alone or in combination with a condensation trap may be used to reduce or eliminate the effects of EGR condensate.
The above advantages, and other advantages, objects, and features of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.